The blades and vanes used in the turbine section of a gas turbine engine each have an airfoil section that extends radially across an engine flowpath. During engine operation the turbine blades and vanes are exposed to elevated temperatures that can lead to mechanical failure and corrosion. Therefore, it is common practice to make the blades and vanes from a temperature tolerant alloy and to apply corrosion resistant and thermally insulating coatings to the airfoil and other flowpath exposed surfaces. It is also widespread practice to cool the airfoils by flowing a coolant through the interior of the airfoils.
One well known type of airfoil internal cooling arrangement employs three cooling circuits. A leading edge circuit includes a radially extending impingement cavity connected to a feed channel by a series of radially distributed impingement holes. An array of "showerhead" holes extends from the impingement cavity to the airfoil surface in the vicinity of the airfoil leading edge. Coolant flows radially outwardly through the feed channel to convectively cool the airfoil, and a portion of the coolant flows through the impingement holes and impinges against the forwardmost surface of the impingement cavity. The coolant then flows through the showerhead holes and discharges over the leading edge of the airfoil to form a thermally protective film. A midchord cooling circuit typically comprises a serpentine passage having two or more chordwisely adjacent legs interconnected by an elbow at the radially innermost or radially outermost extremities of the legs. A series of judiciously oriented cooling holes is distributed along the length of the serpentine, each hole extending from the serpentine to the airfoil external surface. Coolant flows through the serpentine to convectively cool the airfoil and discharges through the cooling holes to provide transpiration cooling. Because of the hole orientation, the discharged coolant also forms a thermally protective film over the airfoil surface. Coolant may also be discharged from the serpentine through an aperture at the blade tip and through a chordwisely extending tip passage that guides the coolant out the airfoil trailing edge. A trailing edge cooling circuit includes a radially extending feed passage, a pair of radially extending ribs and a series of radially distributed pedestals. Coolant flows radially into the feed passage and then chordwisely through apertures in the ribs and through slots between the pedestals to convectively cool the trailing edge region of the airfoil.
Each of the above described internal passages--the leading edge feed channel, midchord serpentine passage, tip passage and trailing edge feed passage--usually includes a series of turbulence generators referred to as trip strips. The trip strips extend laterally into each passage, are distributed along the length of the passage, and typically have a height of no more than about 10% of the lateral dimension of the passage. Turbulence induced by the trip strips enhances convective heat transfer into the coolant.
The above described cooling arrangement, and adaptations of it, have been used successfully to protect turbine airfoils from temperature related distress. However as engine designers demand the capability to operate at increasingly higher temperatures to maximize engine performance, traditional cooling arrangements are proving to be inadequate.
One shortcoming of a conventionally cooled airfoil is its possible unsuitability for applications in which the operational temperatures are excessive over only a portion of the airfoil's surface, despite being tolerable on average. Locally excessive temperatures can degrade the mechanical properties of the airfoil and increase its susceptibility to oxidation and corrosion. Moreover, extreme temperature gradients around the periphery of an airfoil can lead to cracking and subsequent mechanical failure.
Another shortcoming is related to the serpentine passage. A serpentine passage makes multiple passes through the airfoil interior. Accordingly, it takes more time for coolant to travel through a serpentine than to travel through a simple radial passage. This increased coolant residence time is usually considered to be beneficial since it provides an extended opportunity for heat to be transferred from the airfoil to the coolant. However the increased residence time and accompanying heat transfer also significantly raise the coolant's temperature as the coolant proceeds through the serpentine, thereby progressively diminishing the coolant's effectiveness as a heat sink. If the engine operational temperatures are high enough, the diminished coolant effectiveness can offset the benefits of lengthy coolant residence time.
A third shortcoming is related to the desirability of maintaining a high coolant flow velocity, and therefore a high Reynolds Number, in internal cooling passages perforated by a series of coolant discharge holes. The accumulative discharge of coolant through the holes is accompanied by a reduction in the velocity and Reynolds Number of the coolant stream and a corresponding reduction in convective heat transfer into the stream. The reduction in Reynolds Number and heat transfer effectiveness can be mitigated if the cross sectional flow area of the passage is made progressively smaller in the direction of coolant flow. However a reduction in the passage flow area also increases the distance between the perimeter of the passage and the airfoil surface, thereby inhibiting heat transfer and possibly neutralizing any benefit attributable to the area reduction.
A fourth shortcoming affects the airfoils of blades, but not those of vanes. Blades extend radially outwardly from a rotatable turbine hub and, unlike vanes, rotate about the engine's longitudinal centerline during engine operation. The rotary motion of the blade urges the coolant flowing through any of the radially extending passages to accumulate against one of the surfaces (the advancing surface) that bounds the passage. This results in a thin boundary layer that promotes good heat transfer. However this rotational effect also causes the coolant to become partially disassociated from the laterally opposite passage surface (the receding surface) resulting in a correspondingly thick boundary layer that impairs effective heat transfer. Unfortunately the receding passage surface may be proximate to a portion of the airfoil that is subjected to the highest temperatures and therefore requires the most potent heat transfer.
It may be possible to enhance the heat transfer effectiveness in a conventional airfoil by providing a greater quantity of coolant or by using coolant having a lower temperature. In a gas turbine engine, the only reasonably available coolant is compressed air extracted from the engine compressors. Since the diversion of compressed air from the compressors degrades engine efficiency and fuel economy, extraction of additional compressed air to compensate for ineffective airfoil heat transfer is undesirable. The use of lower temperature air is usually unfeasible since the pressure of the lower temperature air is insufficient to ensure positive coolant flow through the turbine airfoil passages.
Improved heat transfer can also be realized by employing trip strips whose height is greater than 10% of the passage lateral dimension. However this approach is unattractive for rotating blades since the trip strips are numerous and the aggregate weight arising from the use of enlarged trip strips unacceptably amplifies the rotational stresses imposed on the turbine hub.